1. Field of the invention.
This invention pertains to that field of art concerning electronic switching regulators and more particularly to a circuit for reducing turn off time of the main switching transistor used in these regulators.
2. Description of the prior art.
It is common knowledge that in switching transistor applications the turn on and off times are the periods of greatest power dissipation within the transistor. Due to thermal considerations it is imperative that these times be reduced as much as possible. Because fast trigger driver transistors are now available which can supply ample current quickly to the base/emitter junction of the main switching transistor, the turn on time is no longer a serious problem. This is not so as to the turn off time. Very high currents are customarily switched during conduction and the main switching transistor is in deep saturation. Collector current flows for some time after the termination of the cut off pulse. This is due to the fact that the large number of minority carriers necessary for saturation require some finite time to be swept clear of the collector/base junction. Various attempts have been made in the past to speed up the turn on and turn off times. Speed up capacitors have been used to bypass the base drive resistor to speed up turn on and transistor design has undergone modification. No one has as yet invented a workable method of speeding up turn off time. Due to the fact that any pulses fed back to the main switching transistor must of necessity be regenerative to accomplish this end, oscillations frequently occur. This invention provides the necessary filter and storage inductance in the main switching transistor output as well as speeding up transistor turn off and at the same time avoids oscillations.